Weft feeding devices for weaving looms are apparatuses which are arranged between the loom and the thread reels which feed the weft to the loom, to perform the function of unwinding the thread off the reels and hence make it available to the weft insertion devices, keeping the thread tension within acceptable levels during the entire weft insertion operation, and hence avoiding the abrupt tension peaks in the thread which occur instead upon weft insertion in looms without weft feeders. This object is achieved through the presence, in the weft feeder, of a winding assembly which regularly and at a lower average speed takes the weft thread from the reels, accumulating it in successive coils on a stationary cylindrical drum whereon it hence forms a thread stock. Such stock is then collected, discontinuously and at high speed, by the weft insertion devices (launch nozzles or grippers) of the loom.
The weft feeder is an apparatus which has been in use in weaving looms for many years now, in particular since modern high-speed looms have been introduced, wherein the direct feeding from the reels has never been technically possible. During its evolution over the years, in addition to the basic functions recalled above, the weft feeder has acquired additional control functions which allow to verify the constant presence of thread in the critical points of the weft feeder, to adjust the amount of thread accumulated in the stock and the distance between the individual coils, to brake the outgoing thread to limit the dynamic effects determined by the abrupt acceleration during its collection by the weft insertion devices, to measure the length of the thread portion collected by the insertion devices, and hence to stop thread collection as soon as a predetermined length thereof has been supplied.
These different functions are obtained due to the presence, aboard the weft feeder, of a processing unit which operates on the basis of sophisticated algorithms, starting from electric signals for the detection of the thread presence/absence in correspondence of the above-said critical points of the apparatus. These electric signals are currently obtained by means of pairs of emitting/receiving optic sensors arranged on the weft feeder so that the path of the optic radiation between an emitting sensor and a receiving sensor intercepts the thread path in a desired control position. Depending on the type of path of the optic radiation, and consequently of the positioning of the LED optic sensors on the weft feeder, current weft feeders divide into two categories.
In a first weft feeder category, both emitting sensors and receiving sensors are arranged on a support arm which projects from the main body of the weft feeder and extends parallel to the lateral surface of the drum, and the path of the optic radiation between each pair of sensors is obtained through a respective reflecting surface fastened to the lateral surface of the drum which faces said support arm, in a carefully preset position and angle.
In a second, more recent weft feeder category, the emitting sensors are instead arranged precisely on the lateral surface of the stationary drum, while the receiving sensors remain in the above already described position on the support arm. In this second weft feeder category one has the advantage that the optic radiation emitted by the emitting sensors is directly intercepted by the receiving sensors, and hence the relative electric signal corresponding to the presence/absence of such optic radiation (which signal is determined by the absence or presence, respectively, of a thread through the path of the optic radiation) is much stronger and more stable with respect to the one of the preceding reflection system. On the other hand, this second weft feeder category has the disadvantage of not being able to supply the energy necessary for activating the emitting sensors through standard electric cables since, as it is well known to people skilled in the field, the stationary drum of the weft feeder is kept in a stable position on the rotary shaft of the weft feeder solely through magnetic means to allow that a rotor, integral with said rotary shaft and apt to perform the winding of thread coils on the drum, be housed between the weft feeder body and the drum. There is hence no fixed mechanical connection of the drum to the weft feeder body along which a conventional electric connection may pass and hence the electric supply of the emitting sensors must be obtained through independent means arranged inside the drum (batteries), or through induction supply assemblies comprising a pair of electric coils housed on the weft feeder body and on the drum, respectively.
In the most recent years, as the above-said different performances of weft feeders became increasingly more complete and reliable, a new feature of these apparatuses has become important, i.e. the flexibility of application thereof on the most diverse weaving machines. As a matter of fact, this application flexibility is highly in demand with weavers, who can thus free themselves from the need to arrange various types of weft feeders depending on the different weaving machines they have to be intended for or on the different types or colours of threads used for weft formation.
In fact, in general high operation flexibility of the weft feeder upon variation of the conditions of the supplied thread is required by weavers, both as far as the thread count and the thread colour is concerned. Specifically, it is required for the weft feeder to be able to operate regularly also in the presence of very thin threads or of darkly-coloured or clear or highly reflective threads, which threads are hence harder to be optically detected.
In particular, as far as measuring weft feeders are concerned—i.e. those weft feeders which are capable of measuring the amount of thread collected by the insertion devices and to halt the collection thereof upon reaching a preset length, today mainly used in air looms and water looms—it is highly appreciated by users the fact that such length may be varied within a wide range of measures, depending both on the type of weaving machine and on the height of the individual fabric being woven. To reach this aim, in addition to obviously varying the number of thread coils accumulated on the drum which are released for each insertion, it is already known to form the drum itself through multiple, independent cylinder sectors, the radial position of said sectors being manually adjustable between a minimum-radius position and a maximum-radius position. However, while this last (maximum-radius) condition may be determined at will during the design stage of the weft feeder, the minimum-radius condition is instead naturally limited by the bulk of the devices which must be housed inside the drum. In the category of direct optic-sensor weft feeders which is targeted by the present invention, among these devices there are hence also the emitting sensors and the corresponding induction supply circuit of said sensors, electric coils included, and this has caused, precisely because of the additional bulk determined by these devices, this category of weft feeders to have so far a smaller flexibility of use, in the field of short weft lengths, compared to weft feeders with reflexion optic sensors.